Underwater thruster
An underwater thruster is a configuration of marine propellers and hydraulic or electric motor built into, or mounted to Underwater Robot as a propulsion device. These give robot move and maneuverability against sea water resistance. the main difference between underwater thrusters and marine thrusters is the ability to works under heavy water pressure, sometime up to full ocean depth.
Types of underwater thrusters
Underwater thrusters can be divided in two main groups, Hydrolic thrusters and Electric thrusters. Electric thrusters mainly uses on battery operated underwater robot such as AUVs, submarines and electric ROVs. But Hydrolic thrusters mainly uses on work class hydraulic ROVs. Hydraulic thruster technology are older than electrical one, more rugged and their weight to thrust ratio are higher than electric thrusters but maintenance and their piping issues cause some dissatisfaction with users. Thanks to developing PMSM electric motors (wrongly knowing Brush-less Dc Motor in market), the electric thrusters find their ways to be more popular in newly designed products. Electric thruster weight to thrust ratio are higher than hydraulic one, but counting accessories including valves, hydraulic pumps, pipes joints, etc. makes them evens heavier than electric thrusters. Electric thrusters in beginning stages had some unreliability with electronic controllers, but power electronic developments made them more rugged these days, some models can be found on the market with +10 years service life and many years warranty.
Electric Underwater thruster variation problems and parts
Each underwater thruster has includes some main sections as follows:
- Electric Motor: Electric motor is the main section of electric thrusters and move the propeller. Modern underwater thrusters usually use brush-less Permanent Magnet Synchronous Motor (PMSM). In some low quality thrusters Brush-less Dc Motor is used, the gain is lower price and the penalty is some percent of efficiency. In most modern designs, House less PMSM motors (mainly produced Kolmorgen) are used to reduces the weight and increases the thermal efficiency. The gain is to reduces the weight, increase the motor power to weight ratio, and the penalty is higher assembly cost.
- Gearbox: to matches propeller torque with motor torque some companies uses a gearboxes. At most of the time to reduced the weight and volume of the thruster, the gears assembled inside the thruster shell directly. In this case the thruster shell is used as gearbox housing. Although in this way the weight decreases considerably, but repairs become difficult for repair men as the spare part can not be found on the normal market.
- Direct driving: In some modern design which using PMSM motors, the ratio of motor torque to its diameter is so high that motor can rotate the propeller without gearbox. In direct drive underwater thrusters, motors are heavier than the motors that is used in the non-direct drive thrusters, but removing the gearbox compensate this mater. Direct drive thrusters have higher reliability, lower noise, and higher efficiency, but its prices are higher than non-direct one.
- Motor driver and Electronics: Brush-less motors need some Electronics to be commutated and control their speed. In early stages the drivers has some unreliability in their design and it makes user unhappy when the compare it with highly reliable hydraulic thrusters. Nowadays developing power electronic technology makes the motor derive more efficient, reliable,cheap and small, to be fitted at the end of the motor directly. In modern design the motor controller are able not only to control the propeller RPM, but also they are able to control the thrust force in applications that need close control on their positioning.
- Shafting and sealing: Keeping propeller in right places and make it reliable in case of hitting with external parts especially fishes or fishing nets are one of the main concern in all kind of thrusters. Lots of failure have been reported due to this problem. Some companies try to solve it by using magnetic coupling and totally avoiding and rotary sealing. Yes the reliability of the sealing and shafting increase a lot, but they loose in thruster efficiency due to limited torque transfer capability of magnetic coupling and they solve it by using high speed low torque propeller. In most models the efficiency is as low as 25% which is very low for underwater thrusters. also magnetic bearing makes the propeller to be rotated on the outer shell surfaces on a layer of water as a lubricant. This design may reduced the bearing life considerably in non clear water. Some other companies gained advanced design by using tapered bearings and multiple sealing system as redundancy. In this advanced design if main seal (usually ceramic seal) fails the others keep the motor safe and thruster can continue to its operation.
- Propeller: propeller is the main part to changes the rotation to thrust. Selection of the right propeller may change the performance of the thrusters a lot. Each application's hydrodynamic Load Line need a special designed propeller, if the efficiency needs to be maximized. But there is lack of standard off the shelf propeller variation on the market and therefore it is impossible to order the thruster with the best efficiency propeller. Some companies accept to design and develop custom propellers, but their price are really high. The other companies tries to offer as many propeller on the market as an option of their thrusters and lets the user select the right one using performances charts of their thrusters.
- Nozzle: Nozzle are basically uses with heavy load, low speed thrusters. Most ROVs includes in this type of hydrodynamic loads. In high speed light load robots, such as AUVs, UUVs and submarine, usually the thrusters does not have any nozzle.
- Handle: Underwater thrusters vibrates due to non-uniform load on propeller blades. also they suffer from heavy impacts of fish propeller collision. To conquer all of this problem, handles gets heavy and wide area witch may considerably blockages the incoming flow to the propeller that reduces the thruster performance. All of this matters make the handle an important section of the thrusters. There are some solutions are on the different products on the market. Some company offer their thrusters without a handle and they let the user to develop their own solutions. But the most efficient solution on the market is to uses of the nozzle stiffener as a thruster handle. It need lots of engineering effort to develop this type of handle, but outcome is a light weight, low flow blockage, impact and vibration resistance thruster.
- Shell: thruster shells usually must be resistant to seawater corrosion. There are two version of shells hard anodized aluminium and stainless steal 316. steel are heavier more expensive and more rubberized. Aluminium are lighter cheaper.
- Electric Connector: electric connectors of thrusters are of the main parts of underwater thrusters. Fortunately there are lots of off the shelf solution on the market (SEACON) that makes using them more reliable and easy.
Performances
There are lots of parameters that affects the underwater thrusters considerably. Under the sea level, energy become invaluable as it is difficult to transfer it (ROVs) or to store it (AUV, UUV, Submarine), Then its very important to have the maximum efficiency. Motor driver, Electric motor, Shafting, Sealing, Propeller, Nozzle and thruster outer geometry and surface all affects the efficiency.
- matching the propeller load with motor torque: One of the very difficult design problem of underwater thrusters is to matches the propeller load line with the motor power line. If it does not happens the overall efficiency of thruster will fall well below maximum or a small percents of, motor power will be used.
- Using right propeller: Propeller diameter, Pitch ratio and Type are very important to have the maximum performance. Lots of care and engineering must be done before final order of thruster to have the right choice.
- Using low THD motor and Driver: PMSM motors have some efficiency problem with THD (Total Harmonic Distortion). Low THD motors and drivers are available on the market (Kolmorgen)but its prices are considerably higher than normal low efficiency one. Only high-tech Thrusters on the market using this type of motor and driver (Lian Innovative).
- Streamlined Thruster Shell: Manufacturing of streamlined body and handle have a considerable effects on the efficiency. And manufacturing of curves in this type of geometry are expensive.
See also
External links
- Advanced Electric Underwater Thruster, lian innovative is the manufacturer of widest range of underwater thrusters.